This invention relates generally to processes or methods for recovering minerals from brine and is particularly concerned with recovering crystalline magnesium silicates from geothermal brine formed during the production of energy from a geothermal fluid.
General processes by which geothermal brine can be used to generate electric power have been known for some time. Geothermal brine from a producing well, having a wellhead temperature over about 180xc2x0 C. and a wellhead pressure over about 400 psig, for example, can be flashed to a reduced pressure to convert a portion of the brine into steam, which is then fed to a conventional steam turbine-type power generator to produce electricity.
It is generally known that the solubility of most dissolved ions in geothermal brine decreases with a decrease in brine temperature. If dissolved ions are present near their saturation concentration in the brine, a significant reduction in the temperature of the system can result in supersaturation and precipitation of a portion of these saturated ions. Precipitates can combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the brine is confined.
An especially troublesome component of hot brine is silica, which may be found near saturation concentrations in the form of silicic acid oligomers. These species tend to precipitate out of solution at almost every stage of processing geothermal fluids to recover their energy, either as substantially pure silica or as tightly adherent metal-silica scale having little value. The resultant deposits or scale can plug conduits, injection wells, and the subterranean formation in the vicinity of the injection wells and quickly foul conventional heat-exchangers. Thus, these deposits must be controlled in order to avoid frequent process shutdowns to remove silica-containing scale.
It has been shown that acidifying the brine can significantly reduce its scaling tendencies and allow for a much longer continuous operation of the energy recovery process. However, the acid adds an additional expense to the process and can also cause corrosion and handling problems, such as the introduction of oxygen into an otherwise oxygen-free brine, the embrittlement of equipment, and problems associated with re-injection into a subterranean formation. Thus, alternative methods of controlling silica scale deposition without significantly increasing the cost of power production or creating other brine-handling problems are needed.
In accordance with the invention, it has now been found that silica precipitation from brine can be controlled or prevented, especially during brine processing to recover energy therefrom and the subsequent re-injection into a subterranean formation to replenish underground aquifers, while at the same time producing a valuable byproduct by using the brine to synthesize crystalline magnesium silicate minerals, such as kerolite. The silica-containing brine, which is usually a geothermal brine produced during extraction of energy from geothermal fluids removed from a subterranean formation, is mixed with a magnesium-containing compound, the pH of the resultant mixture is adjusted to a value between about 8.0 and 14, and a magnesium silicate is crystallized from the mixture at a temperature between about 50xc2x0 and 200xc2x0 C. The magnesium silicate, which may have use, for example, in the manufacture of pharmaceuticals, cosmetics and/or drilling muds, is recovered and typically sold to recoup some of the cost of processing the brine, usually to extract energy therefrom. The treated brine may then be further processed or re-injected into a subterranean formation without problems caused by silica precipitation.
The particular type of crystalline magnesium silicate produced in the process or method of the invention typically depends on the amount of the magnesium-containing compound used, the pH of the mixture of brine and magnesium compound and the crystallization temperature. For example, kerolite, which is a crystalline magnesium silicate clay frequently having the chemical formula Mg3Si4O10(OH)2.H2O and used in the pharmaceutical and cosmetics industries, is synthesized by using enough of the magnesium-containing compound such that there is about a xc2xe mole of magnesium for every mole of silica in the mixture of brine and magnesium compound, adjusting the pH of the mixture to a value between about 9.5 and about 10.5, and crystallizing the mixture at a temperature between about 100xc2x0 C. and about 170xc2x0 C. Other crystalline magnesium silicates, including specialty clays, can be made by changing the synthesis conditions and the amounts of reactants. Examples of such magnesium silicates include, among others, sepiolite, talc, stevensite, polygorskite, saponite, and hectorite.